Absorption spectroscopy is one of the most quantitative methods for identifying unknown gases, liquids, and aerosols in either the laboratory or real world applications. For instance, in the infrared portion of the spectrum, many molecules contain a “fingerprint,” a unique pattern of optical transitions that are distinct due to the compounds chemical moiety (OH, CH, CO). The primary issues with using absorption spectroscopy as an everyday tool for identifying environmental contaminates are that large spectral bandwidths with good resolution are needed and high sensitivity is required. Typically contaminates of the greatest concern/interest are very dilute in the environment so some manner of increasing the sensitivity of direct absorption methods are needed for sensor development.
Cavity-ring-down based spectrometers, which use optical resonators to increase the effective path length of the absorbing medium, are now routinely used to monitor green house gasses. However they are generally limited to measuring absorption at extremely narrow spectral regions associated with the laser light source used. Modern day state-of-the-art spectrometers attempt to overcome the issues of spectral resolution (selectivity) at the cost of bandwidth. Generally, high-resolution laser light sources are used that are resonant with an atomic or molecular transition of interest, but these sources are not broadly tunable. When using a broad light source the frequency resolution is limited by the spectrometer.